The present invention relates to inhibitors of the Tec family tyrosine kinases, and particularly, inhibitors of Emt. The Tec family kinases include Emt [expressed mainly in T cells; Gibson, S. et al., Blood 82, 1561-1572 (1993)], Txk [T-cell expressed kinase; Haire, R. N. et al., Hum. Mol. Genet. 3, 897-901 (1994)], Tec [tyrosine kinase expressed in hepatocellular carcinoma cells; Mano et al., Oncogene 5, 1781-1786 (1990)], Btk [Bruton's tyrosine kinase; Vetrie, D. et al., Nature 361, 226-233, (1993)], and Bmx [bone marrow kinase, X-linked; Tamagnon, L. et al., Oncogene 9, 3683-3688 (1994)].
Mammalian immunity relies on the activation of T cells upon antigen presentation. The molecular mechanisms of T cell activation are initiated by the sequential activation of three distinct classes of non-receptor protein tyrosine kinases (PTK) following the engagement of the T cell antigen receptor (TCR). These three classes of PTK are the Src family kinases (Lck and Lyn), the Syk family kinases (ZAP-70 and Syk), and the Tec family kinases (Emt, Txk, and Tec). Inhibition of one or more of these kinases will impede the initiation signals and block T cell activation following antigen presentation. Thus, small molecular weight inhibitors of these kinases can be applied to treat the diseases that are associated with unwanted T cell activation.
Emt, also known as Itk (Interleukin-2-inducible T cell kinases) or Tsk (T-cell-specific tyrosine kinase), is expressed solely in T, natural killer, and mast cells. Emt is tyrosine-phosphorylated and activated in response to cross-linking of TCR, CD28, or CD2; and has been implicated in thymocyte development and the activation of T cells through TCR and CD28 engagement. Inside the cells, Emt is regulated by membrane recruitment followed by Lck phosphorylation and autophosphorylation. Emt is recruited to the membrane rafts for Lck phosphorylation through the interaction between the pleckstrin homology (PH) domain of Emt and the membrane lipid, phosphotidylinositol (3,4,5)-triphosphate [Bunnell et al., J. Biol. Chem. 275, 2219-2230 (2000)].
Gene knockout studies reveal that mice lacking Emt have decreased numbers of mature thymocytes, especially CD4+ T cells. The T cells isolated from such mice are compromised in their proliferative response to allogeneic MHC stimulation, and to anti-TCR/CD3 cross-linking [Liao X. C. and Littman, D. R., Immunity 3, 757-769 (1995)]. These T cells also exhibit defective PLCγ1 tyrosine phosphorylation, inositol triphosphate production, Ca2+ moblization, and cytokine production (such as IL-2 and IFNγ) in response to TCR cross-linking [Schaeffer, E. M. et al., Science 284, 638-641 (1999)]. This genetic evidence indicates that Emt activity plays a requisite role in TCR signal transduction; and selective inhibition of Emt should have immunosuppressive, anti-inflammatory, and anti-proliferative effects. In addition, Emt-deficient mice are unable to establish functional Th2 cells (the IL-4 producing cells) and such mice are unable to clear parasitic infections depending upon a Th2 response [Fowell, D. J. et al., Immunity 11, 399-409 (1999)]. This observation also suggests that Emt may be an attractive target for modulating dysregulated allergic pathways mediated by Th2 cells.